gpu: drm: msm: add event to event_list after register is successful

Add event to event_list after msm_register_event is successful to avoid
use-after-free vulnerability.

Change-Id: I34fb39c99051978cbab64a852851964691a5ea9e
Signed-off-by: Ping Li <pingli@codeaurora.org>
Signed-off-by: Venkata Prahlad Valluru <vvalluru@codeaurora.org>